This invention relates to methods for fabricating semiconductor devices, and more particularly to, methods for mass producing high density field effect transistors having gate lengths that are substantially shorter than those which are now commercially available. Those gate lengths are in the range of 3.5-4.5 micrometers. This is evidenced for example, by a recent article entitled "The Race Heats Up in Fast Static RAMs", published in Electronic, Apr. 26, 1979 at pages 125-135.
In the past, many research projects have been carried out to devise a mass production method for fabricating field effect transistors of smaller dimensions. But to date, due to various technological problems, the disclosed devices remain in the laboratory. A current review of the state of this technology, which considers gate lengths of approximately 1.0 micrometers, is presented in a series of eight papers in the IEEE Journal of Solid State Circuits, Vol. SC-14, No. 2, April 1979, at pages 240-301.
One important requirement that must be met in any mass production LSI/VLSI process is that the transistors which are thereby produced have a very low probability of being defective. This is because, one defective transistor in an LSI/VLSI chip generally will make the entire chip inoperative. Thus for example, if each chip contains 100,000 transistors, then the probability that any one of those transistors will be produced with a defect must approach 1/100,000 to obtain resonable production yields. On the other hand, the above requirements need not even be considered when only a few short gate length transistors or very small scale integrated circuits are produced for evaluation in the laboratory, such as those which are discussed in the above referenced IEEE papers.
Therefore, a primary object of the present invention is to provide an improved method of fabricating miniature field effect transistors.
Still another more specific object of the invention is to provide a method of mass producing 2.5 micrometer gate length field effect transistors in high density LSI/VLSI chips with reasonable yields.